In recent years, there is an increasing need for large and high-rise buildings, and a construction machine or industrial machine, such as a crane used for construction, must be large and have high-performance, in order to shorten construction time and reduce construction cost. Therefore, the strength of a steel plate used for the construction machine or the industrial machine has been increased, and, an ultrahigh-strength steel plate, having the tensile strength more than 1100 MPa, is currently being.
In the prior art, when welding a steel plate, manual metal arc welding, gas shielded arc welding or submerged arc welding, etc., has been used. However, in recent years, in order to improve the efficiency of the welding of the steel plate, the use of laser hybrid welding, wherein a laser beam and gas shielded arc welding are combined, has been considered.
As a method for the laser hybrid welding, a method wherein a pool is formed by the gas shielded arc welding and the laser is irradiated on the pool, or, a method wherein the laser is irradiated on the steel plate so that the steel plate is melted by high-density heat input and the gas shielded arc welding is performed thereon, may be possible.
When the laser hybrid welding is used to weld the steel plate, a deeper weld penetration is obtained in comparison to the conventional welding. Therefore, in the laser hybrid welding, the single pass welding can be performed when the thickness of the steel plate is not more than 12 mm, whereby the steel plate may be welded effectively.
As a method for welding the steel plate by using the laser hybrid welding, techniques of Patent Literatures 1 and 2 have been proposed.
PLT 1 discloses a method for producing an ultrahigh-strength welded steel pipe including the steps of: providing a steel plate having a tensile strength over 800 MPa, which has a microstructure corresponding to a mixed structure of ferrite, plus tempered martensite and lower bainite; forming the steel plate into a piped-shape; and welding butted parts by laser hybrid welding.
Further, PLT 2 discloses a method for producing an ultrahigh-strength and highly-deformable welded steel pipe including the steps of: providing a steel plate having a tensile strength over 900 MPa, which has (ferrite plus bainite), (ferrite plus martensite) and (ferrite plus bainite plus martensite), one of which having an area fraction equal to or more than 90%; forming the steel plate into a pipe-shape; and welding butted parts by laser hybrid welding.